Yes that's true, however, diesel and petrol works different. On diesel the
amount of air is always the same while the amount of fuel is changed
(quality controlled), while on a petrol the quality of the mixture is the
same while the amount of it changed (quantity controlled). Maybe it has been
improved a bit for modern engines using injectors and turbo chargers for
both type, but the principal should be still the same. It means that for a
diesel the higher the rotation speed the higher power coming out from, while
a petrol has nice a curve, where there is a well seen optimum speed when the
torque is the highest - till and from that point the outcome power is less
and less even though the rotation is higher. With a diesel the torque is not
much less at slow rotation and it graduately increasing with the speed till
it falls apart. So when we talk about horse power we have to keep in mind
that for a petrol they give this number for that optimal rotation speed -
you can see that number in the brochure sometimes.

However, because of the same engine size of petrol spins much faster than
diesel, petrol engines are much "powerful". My engine for example spins
around 1500 rpm when driving at 90km/h (55mph), but it has an automatic gear
box which is very comfortable but the ratio is far from the optimal. In the
opposite, my father's car (which is a Ford Mondeo - and Ford has a
reputation of low rpm engines) spins almost the double at that speed with a
manual gear. It can easily reach 180 km/h where the engine spins somewhere
around 6k rpm - that would kill my diesel engine instantly :-)

Tamas



On 1/16/08, Gerhard Fiedler <lists@connectionbrazil.com> wrote:
>
> Apptech wrote:
>
> > Useful rule of thumb.
>
> For some values of "useful" :)
>
> >     Torque x RPM = power
> >
> >             Torque in kg.m
> >             Power in Watts.
>
> FWIW, I don't know any measurement system in current use where torque is
> measured in kg m. There has been one in use where torque was measured in
> kp
> m (which would have a numeric value close to what kg m could mean if it
> were torque :), but I'm not sure there are many on this list who have
> heard
> of it. In any case, there's probably no place where you find torque
> specified or graphed in kg m.
>
> Currently, at least in the realm where one talks about kg and m, torque is
> measured in N m. And in that system, the rule is not really useful.
>
> > Based on
> >
> >     Power (Watt)  = 2 x Pi x Torque (kg x g x m) x RPM/60 x
> >
> > The 2Pi.g and the 60 ALMOST cancel.
>
> While this is correct, the thing is that after you cancel out the g (which
> is part of the torque), the "torque" that remains is no torque anymore.
> Not
> in units, not in numbers, not in meaning.
>
> So given that one has to do some scaling anyway, it's possibly more useful
> to give the (approximate) scaling factor:
>
>   P [W] = speed [rpm] * torque [Nm] / 10
>
> or
>
>   P [hp] = speed [rpm] * torque [Nm] / 7000
>   P [hp] = speed [rpm] * torque [lbf ft] / 5200
>
> Gerhard
>
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